Rotary well drilling practices include the use of drill strings with drilling fluid pumped down the drill string bore. On long drill strings, communication with the downhole system from the earth surface is difficult, because electric conductors are hard to install and maintain while drilling is in progress.
In recent years, it has become common practice to measure downhole parameters of interest with sensors and convert the resulting information into electric signals, convert the signals to drilling fluid pressure pulses in the drilling fluid in the pipe string bore, and to detect and decode the pressure pulses at the earth surface to recover the information transmitted.
The downhole apparatus that responds to electric signals and generates pressure pulses has been troublesome. Electric power has been used to operate solenoids to operate valves to generate drilling fluid pressure pulses. The generation of electric power has been a problem, and the valve operating gear has been costly and prone to failure.
Apparatus is needed that will cause cyclic pressure changes in the drilling fluid stream, unless action is taken to stop the pulse generating apparatus. Hardware needed to stop a self-excited pulse generator can be much simpler and consume less electric power than hardware required to cause and control pulse generation.